1. Field of the Invention
The present invention relates in general to a fluid-filled elastic mount for damping or isolating vibrations based on flow of a fluid contained therein. More particularly, the present invention is concerned with such a fluid-filled elastic mount which is capable of exhibiting different vibration damping or isolating characteristics depending upon the type of the vibrations applied thereto, assuring an excellent damping or isolating effect for a wide frequency range of input vibrations.
2. Discussion of the Prior Art
As one type of vibration damping devices such as an engine mount for a motor vehicle, there is known a so-called fluid-filled elastic mount which includes a first and a second support structure that are spaced apart from each other and elastically connected to each other by an elastic body interposed therebetween. The fluid-filled elastic mount has a pressure-receiving chamber and a variable-volume equilibrium chamber which are filled with a suitable non-compressible fluid, and an orifice passage which permits flow of the fluid therethrough between the two fluid chambers. The pressure-receiving chamber is partially defined by the elastic body, and a pressure of the fluid in this chamber changes upon application of vibrations. The equilibrium chamber is partially defined by a flexible diaphragm which is elastically deformable to accommodate a volumetric change of this chamber. The fluid-filled elastic mount of the above type damps the input vibrations, based on resonance of a mass of the fluid flowing through the orifice passage, more effectively than an elastic mount which relies only upon the elasticity of the elastic body for damping the vibrations.
Generally, the elastic mount is required to exhibit different vibration damping or isolating characteristics depending upon the type of the vibrations applied thereto. For example, the elastic mount when used as a vehicle engine mount is required to exhibit high damping capability with respect to low-frequency large-amplitude vibrations, such as engine shake and bounce, and to provide a sufficiently reduced dynamic spring constant with respect to middle-frequency vibrations such as engine idling vibrations and high-frequency small-amplitude vibrations such as booming noise.
However, the fluid-filled elastic mount constructed as described above can provide a sufficiently high damping effect based on the resonance of the fluid mass in the orifice passage, only with respect to the vibrations whose frequencies are in the neighborhood of the frequency to which the orifice passage is tuned. Therefore, it has been extremely difficult for the known elastic mount to effectively damp or isolate two or more types of vibrations or a wide frequency range of vibrations.
To achieve a high damping effect for low-frequency vibrations and a sufficiently low dynamic spring constant for middle-frequency vibrations, there has been proposed a mounting device as disclosed in laid-open Publication No. 58-29517 of unexamined Japanese Utility Model Application, which has first and second orifice passages formed in parallel with each other between the pressure-receiving and equilibrium chambers The first orifice passage is tuned so that the mount can effectively damp the low-frequency vibrations such as engine shake, while the second orifice passage is tuned so that the mount provides a relatively low dynamic spring constant for the middle-frequency vibrations such as engine idling vibrations. The proposed mounting device further has a valve system for selectively opening the first or second orifice passage so that the fluid is forced to flow through the selected orifice passage. To achieve a sufficiently low dynamic spring constant for high-frequency vibrations, on the other hand, there has been proposed a mounting device as disclosed in laid-open Publication No. 57-9340 of unexamined Japanese Patent Application, which has a movable member disposed between the pressure-receiving and equilibrium chambers. This movable member is displaceable or deformable over a given distance, and is adapted to absorb a pressure change of the pressure-receiving chamber upon application of high-frequency vibrations such as booming noise.
In the mounting device having the two orifice passages for damping and isolating low- and middle-frequency vibrations as disclosed in the publication No. 58-29517, it is necessary to incorporate in the structure of the device valve means for selectively opening the orifice passages, and biasing or drive means for actuating the valve means. Accordingly, it is difficult for this mounting device to have enough space for provision of the movable member as disclosed in the publication No. 57-9340, for assuring a relatively low dynamic spring constant with respect to high-frequency vibrations. Even if the movable member is disposed in a limited space in the mounting device, the mounting device cannot provide a sufficiently high vibration isolating effect due to insufficiency of the cross sectional area and the distance of displacement of the movable member.
If the movable member is designed to have an increased cross sectional area and an increased distance of the displacement thereof, to assure a significantly reduced dynamic spring constant for the high-frequency vibrations, the mounting device tends to be large-sized, and suffers from deterioration in the vibration damping or isolating effects based on flow of the fluid through the orifice passages. Namely, even when the mounting device receives the low- and middle-frequency vibrations which should be damped or isolated by the orifice passages, the pressure changes in the pressure-receiving chamber are more or less absorbed by the movable member, whereby the amounts of the fluid flowing through the orifice passages are accordingly reduced. Consequently, the orifice passages are unable to provide a sufficiently high vibration damping or isolating effect. Particularly, upon application of middle-frequency vibrations, such as relatively small-amplitude engine idling vibrations, the mounting device seriously suffers from deterioration of its vibration isolating capability based on the fluid flow through the relevant orifice passage, since pressure changes in the pressure-receiving chamber are largely absorbed by the movable member.
For the reasons as described above, the known elastic mounts cannot provide sufficient vibration damping and isolating effects for a wide frequency range of input vibrations, leaving some room for improvement.